ADM3311EARSZ datasheet(14/24 Pages) AD

Part #	ADM3311EARSZ

Description	15 kV ESD Protected, 2.7 V to 3.6 V Serial Port Transceivers with Green Idle?

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ADM3307E/ADM3310E/ADM3311E/ADM3312E/ADM3315E

Data Sheet

Rev. I | Page 14 of 24

A block diagram of the Green Idle circuit is shown in Figure 26.

Both V+ and V− are monitored and compared to a reference

voltage derived from an on-chip band gap device. If either V+

or V− fall below 7 V1, the oscillator starts up until the voltage

rises above 7.25 V2.

TRANSCEIVERS

START/STOP

V+

V–

SHUTDOWN

CHARGE

PUMP

V– VOLTAGE

COMPARATOR

WITH 250mV

HYSTERESIS

BAND GAP

VOLTAGE

REFERENCE

V+ VOLTAGE

COMPARATOR

WITH 250mV

HYSTERESIS

Figure 26. Block Diagram of Green Idle Circuit

The operation of Green Idle for V+ under various load

conditions is illustrated in Figure 27. Under light load

conditions, C1 is maintained in a charged condition, and only a

single oscillator pulse is required to charge up C2. Under these

conditions, V+ may actually overshoot 7.25 V2 slightly.

OSC

V+

OVERSHOOT

LIGHT

LOAD

7.25V1

7V2

OSC

V+

7.25V1

7V2

OSC

V+

7.25V1

7V2

MEDIUM

LOAD

HEAVY

LOAD

1FOR ADM3310E AND ADM3315E REPLACE WITH 6.5V.

2FOR ADM3310E AND ADM3315E REPLACE WITH 6.25V.

Figure 27. Operation of Green Idle under Various Load Conditions

Under medium load conditions, it may take several cycles for

C2 to charge up to 7.25 V2. The average frequency of the

oscillator is higher because there are more pulses in each burst

and the bursts of pulses are closer together and more frequent.

Under high load conditions, the oscillator is on continuously if

the charge pump output cannot reach 7.25 V2.

Green Idle Vs. Shutdown

Shutdown mode minimizes power consumption by shutting

down the charge pump altogether. In this mode, the switches in

the voltage tripler are configured so V+ is connected directly to

VCC. V− is zero because there is no charge pump operation to

charge C5. This means there is a delay when coming out of

shutdown mode before V+ and V− achieve their normal

operating voltages. Green Idle maintains the transmitter supply

voltages under transmitter idle conditions so this delay does not

occur.

Doesn’t Green Idle Increase Supply Voltage Ripple?

The ripple on the output voltage of a charge pump operating in

open-loop depends on three factors: the oscillator frequency,

the value of the reservoir capacitor, and the load current. The

value of the reservoir capacitor is fixed. Increasing the oscillator

frequency decreases the ripple voltage; decreasing the oscillator

frequency increases it. Increasing the load current increases the

ripple voltage; decreasing the load current decreases it. The

ripple voltage at light loads is naturally lower than that for high

load currents.

Using Green Idle, the ripple voltage is determined by the high

and low thresholds of the Green Idle circuit. These are

nominally 7 V1 and 7.25 V2, so the ripple is 250 mV under most

load conditions. With very light load conditions, there may be

some overshoot above 7.25 V2, so the ripple is slightly greater.

Under heavy load conditions where the output never reaches

7.25 V2, the Green Idle circuit is inoperative and the ripple

voltage is determined by the load current, the same as in a

normal charge pump.

What about Electromagnetic Compatibility?

Green Idle does not operate with a constant oscillator

frequency. As a result, the frequency and spectrum of the

oscillator signal vary with load. Any radiated and conducted

emissions also vary accordingly. Like other Analog Devices

RS-232 transceiver products, the ADM33xxE devices feature

slew rate limiting and other techniques to minimize radiated

and conducted emissions.

1 For ADM3310E and ADM3315E, replace with 6.5 V.

2 For ADM3310E and ADM3315E, replace with 6.25 V.

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